The present invention refers to a process for manufacture of a dendritic polyether comprising a core and at least one branching generation. The process comprises mixing at least one compound having two or more hydroxyl groups and at least one cationic initiator and subjecting at least one hydroxyoxetane to ring opening addition and polymerisation. Said hydroxyoxetane is charged at a rate resulting in and maintaining a reaction below onset at thermal degradation and in an amount resulting in at least one branching generation.
Dendritic polymers belong to a group of polymers, characterised by densely branched structures and a large number of end groups. They are obtained by the polymerisation of for instance ABx monomers, such as AB2 monomers, giving branched structures, with an exponential growth, in both molecular weight and end group functionality, as a function of the degree of polymerisation. Dendritic polymers are either produced by homo or co-polymerisation of one or several monomers, optionally in the presence of a multifunctional core. FIGS. 1 and 2 below illustrate dendritic polymers with and without core.
The use of a multifunctional core is of particular interest, since it allows a higher degree of freedom in the molecular design of dendritic structures. In “Surface Coatings Technology” Volume IV, Chapter V, Bo Pettersson et.al, John Wiley & Sons Ltd, it is taught that two sets of properties can be controlled by the use of a multifunctional core, namely i) the ability to control molecular weight and polydispersity, and ii) the physical properties at a given molecular weight. Examples are given of polydisperse dendritic polyesters, which have been produced with and without the use of a polyalcohol core and it was found that both the polydispersity at a given molecular weight as well as the bulk viscosity could be reduced when a trifunctional polyalcohol core was employed versus homo-polymerisation of the neat monomer 2,2-dimethylolpropionic acid.
Dendritic polymers built up from multifunctional cores are often produced, according to a divergent growth pattern. In the divergent synthesis route, monomers such as ABx monomers (also called chain extenders) first react with a multifunctional core in stoichometric ratio with regard to the number of reactive groups of the core molecule and the number of A groups of the ABx monomer. Obtained reaction product will when AB2 monomers are used have twice the functionality of the starting core molecule. The reaction product according to the first step can then in subsequent steps undergo reactions with further ABx monomers, until the desired molecular weight and/or end group functionality is obtained. The principle for a divergent synthesis route is illustrated in FIG. 3 below.
The term generation is often used for dendritic polymers to describe the number of repetitive steps involved in a synthesis and hence indirectly the molecular weight and end group functionality. In the case of polydisperse products, the term pseudo-generation is sometimes used to clarify that there is an uncertainty as to where exactly the ABx monomers are located within the dendritic structure. The term generation will herein be used as designation for generation as well as pseudo-generation and is defined in accordance with equations 1–3 below.F=f×Xn  Eq. 1N=f(Xn−1)  Eq. 2MHBP=N[Me−MR1]+μF[Mt−MR2]+MI  Eq. 3
Wherein    n=Number of generations or pseudo-generations.    F=End group functionality of dendritic polymer with n generations.    N=Equivalents of chain extenders per equivalent of dendritic polymer with a core functionality f and n generations or pseudo-generations.    MHBP=Molecular weight of end-capped dendritic polymer with n generations.    Me=Molecular weight of chain extender.    MR1=Molecular weight of reaction product formed in reaction between groups A and B (which for polyesters is the molecular weight of H2O).    μ=Fraction of end groups capped with an end-capping group.    Mt=Molecular weight of end-capping group.    MR2=Molecular weight of reaction product formed in reaction between terminal groups and end-capping groups.    Mc=Molecular weight of core molecule.
Dendritic polyethers made by ring opening polymerisation have attracted some interest recently. Dendritic structures made from glycidol have been studied by Vandenberg, E. J., Pol. Sci., Part A: Polym. Chem., 1989, 27, 3113 and Sunder, A. et.al., Macromolecules, 1999, 32, 4240 and dendritc structures from and 3-ethyl-3-hydroxymethyloxetane (trimethylolpropaneoxetane) by Magnusson, H. et.al., Macromol. Rapid Commun., 1999, 20, 453–457.
Ring opening polymerisation of oxetanes is per se known in the art and disclosed and discussed in for instance a number of Patents. U.S. Pat. No. 4,988,797 discloses polymerisation of cyclic ethers having 4 and 5 membered rings in the presence of an intiator consiting of an acid catalyst and an alcohol. The polymers are grown from the alcohol. The acid catalyst is employed at a level of between 0.05 and 0.5 relative to the hydroxyl functionalty of the alcohol. Exemplified polymers are starbranched polyethers. U.S. Pat. No. 6,100,375 teaches an improved method for synthesis of energetic polymers based on cyclic ethers having 4 and 5 member rings. Said polymers are produced in the presence of a solvent by employing a triethoxonium salt and an alcohol, both in co-catalytically amounts, as initiator. Disclosed cyclic ethers are monocyclic compounds having at least one (CH3)nX group, X being —N3, —H, —ONO2, —CL, —CN, —BR or —O(C1–C10 alkyl). Synthesis of ABA triblock polymers and AnB star polymers from cyclic ethers is described in U.S. Pat. No. 4,952,644.
Dendritic polyethers obtained by ring opening polymerisation of hydroxyoxetanes (compounds having one oxetane group and at least one hydroxyl group) offer a rapid process yielding dendritic structures. Dendritic polyethers are furthermore, and contrary to dendritic polyesters, hydrolytically stable, which is of interest in applications wherein an aqeuous and alkaline environment is employed. It is of particular interest to study dendritic polyethers made by ring opening polymerisation of 3-ethyl-3-hydroxymethyloxetane, since the monomer is non-toxic and hence environmentally friendly. The 3-ethyl-3-hydroxymethyloxetane monomer is furthermore only possible to polymerise under cationic conditions, which allows the hydroxyl functionality to be modified under alkaline conditions prior to polymerisation. Said modified product can then be used as co-monomer with for instance neat 3-ethyl-3-hydroxymethyloxetane or other hydroxyoxetane and specific functionalities can thereby be incorporated in the inherent dendritic polymer backbone. Dendritic polymers made from hydroxyoxetanes offer interesting physical properties such as a glas transition temperatures (Tg) in the range of 30–45° C. for polymers made from 3-ethyl-3-hydroxymethyloxetane and low melt viscosities at elevated temperatures.
It has however been found that in order to produce dendritic polyethers from hydroxyoxetanes as defined above, it is difficult to control final molecular weights and obtained polymers differ significantly in properties and chemical conversion, depending on the reaction temperature during polymerisation.
Surprisingly, it has been found that hydroxyoxetane based polymers can suffer from poor thermal stability in air atmosphere and if the reaction temperature is above the onset at thermal degradation, poor final properties will result with low final molecular weights and large amount of residual monomers. Onset is here and hereinafter defined as the left limit of exothermic degradation peak in air at a heating rate of at least 3° C./minute.
A method to overcome above mentioned problems with control of molecular weight of dendritic polyethers from hydroxyoxetanes and the thermal degradation can now quite surprisingly be disclosed. It has been found that it is possible to produce dendritic polyethers from hydroxyoxetanes by employing a divergent synthesis approach, wherein a multifunctional alcohol is used as an initiator core. By using the same theoretical approach as has succesfully been demonstrated for dendritic polyesters made from 2,2-dimethylolpropionic acid and polyalcohol cores, it is possible to control both final molecular weight and polydispersity. It has furthermore been found that by using particular conditions during synthesis it is possible to reach very high chemical conversion and to control the polydispersity, final molecular weight and the like. Said conditions include that a cationic initiator is pre-mixed with an alcoholic core, that at least one hydroxyoxetane monomer is feeded continously to the reaction solution with strict temperature control and the reaction solution is kept at a certain temperature interval below onset at thermal degradation. It has also quite surprisingly been found that the thermal stability of obtained dendritic polyether, having one or more generations built up from at least one hydroxyoxetane and a polyalcohol core, can be significantly improved by neutralising the cationic initiator in stoichiometric amounts with a conventional alkaline species, such as NaOH and the like.
Obtained dendritic polyether disclosed and produced according to the method of the present invention is hence superiour in terms of control of molecular weight and thermal stability compared to what previously have been demonstrated with oxetane monomers.